Thick Cu films (e.g. 5 to 50 μm thick) used as last metal layers in power semiconductor applications exert a strong tensile stress due to the thermal expansion mismatch between Cu and semiconductor substrates such as Si wafers. The tensile is problematic above room temperature and down to −50° C. or −70° C., resulting in a heavy wafer bow. A bowed wafer causes out-of-focus issues for lithography stepper systems and therefore limits the use of additional lithography processes. In addition, the wafer bow increases after wafer thinning. Further processing of bowed wafers proves very difficult.
The thickness of conventional Cu last metal layers is typically below 12 μm to minimize the wafer bowing problem described above. The wafer can be cooled significantly (e.g. <=−70° C.) to reduce the wafer bow. However, thicker Cu layers (e.g. >20 μm) are needed to support advanced device technologies. Also, the effect of wafer cryo-cooling is lost when the wafer is subsequently annealed at temperatures >130° C. because the original wafer bow returns at these temperatures. Such elevated temperatures already occur during a standard prebake step of lithographic resists.